Conventionally, a piezoelectric speaker device has been used which outputs a voice by causing an excitation source section constituted of an piezoelectric element to vibrate a vibration section (e.g., a diaphragm) based on an audio signal. In addition, a compact, lightweight, thin display element has appeared recently. There has been commercialized a portable display device, with such a display element, capable of capturing an image from an external device so as to display the picture. It is desired that such a portable display device be reduced in size to a card as small as a normal credit card or a business card.
For example, Japanese Laid-Open Patent Application No. 244784/2003 (Tokukai 2003-244784; published on Aug. 29, 2003) (Document 1) discloses a speaker-function containing display device which realizes to reduce its size to a size of a card. According to this arrangement, the display device realizes a speaker function of generating a sound without upsizing in such a manner that a vibration section, a substrate constituting a display element based on an audio signal inputted from an external device, is vibrated by an excitation source section constituted of a piezoelectric element.
Recently, in such a piezoelectric speaker and a portable display device, a voice-output-section drive circuit for driving a voice output section based on an audio signal, and a display-section drive circuit for causing a display section to display an image based on a video signal, have been formed of a thin film layer including a polycrystalline silicon thin film formed on an insulative substrate (e.g., a glass substrate), thereby constituting a thin film circuit substrate. Such a thin film circuit substrate is disclosed in Document 1 as well.
Conventionally, a drive circuit in a piezoelectric speaker device and in a display device has been realized by bonding an integrated circuit (IC) on a printed board. Meanwhile, as described above, the thin film circuit substrate can be utilized as a vibration section in a speaker device. Also, in a display device, the thin film circuit substrate makes it possible to integrate a display-section drive circuit into the same substrate that constitutes a display section. This realizes a small, lightweight, thin device, simplifies a manufacturing process, and improves connection reliability.
Meanwhile, Japanese Laid-Open Patent Application No. 50937/1998 (Tokukaihei 10-50937; published on Feb. 20, 1998) (Document 2) describes an arrangement in which an antistatic diode is monolithically provided in a general semiconductor integrated circuit.
However, in a piezoelectric speaker device and a display device which include a conventional drive circuit constituted of a thin film circuit substrate, as well as in the arrangement described in Document 1, no measures are taken against a surge voltage and current.
Therefore, a conventional piezoelectric speaker device using a reverse piezoelectric effect to convert an electrical signal into a stress vibration conducting vibration to air, generates an electromotive force from a positive piezoelectric effect of a piezoelectric element upon receipt of a strong shock caused by an accidental drop. The electromotive force applies an excess voltage (surge voltage) reversely to the voice output section. The surge voltage generates a current (surge current), which may be conducted backward in a route. This might cause damage to the voice-output-section drive circuit on the thin film circuit substrate. Also, in a device having a connection terminal connected to an external device, such as a display device, when a user touches with his/her finger the connection terminal, provided externally to the device, for receiving a video signal from the external device, a surge voltage and a surge current induced from static electricity may be generated. This might cause damage to the display-section drive circuit.
Conventionally, such measures have not been taken for the following reason. As described in Document 2, a general semiconductor integrated circuit can be monolithically provided with an antistatic diode so as to take measures against electrostatic damage. However, since the thin film circuit substrate, presupposed by the present invention, which constitutes a drive circuit of a thin film layer, uses not a bonded-type but a MOS-FET thin film, PN bonding becomes difficult. Therefore, under the existing circumstances, it is impossible to monolithically form an antistatic diode having a predetermined performance.
The antisurge section may be externally provided as a separate component. However, an externally provided separate component necessarily enlarges an entire device, undermining the simplification of the manufacturing process and the connection reliability, both of which are realized by the thin film circuit substrate. This makes impossible to utilize the thin film circuit substrate as a vibration section in a speaker device. In addition, since a separate component mounted externally drives up cost, it becomes difficult, in view of a necessity ratio of the cost and protective measures, to take measures against the enlargement of a device and high cost.